1. Field of the Invention
The present invention relates to an optical waveguide, a semiconductor optical integrated element, and methods for manufacturing the same, more specifically to an optical waveguide and a semiconductor optical integrated element, wherein the reflection of laser beams is reduced at the boundary portion of the optical waveguide, and methods for manufacturing the same.
2. Background Art
As an optical waveguide for a semiconductor laser, there are a ridge-type waveguide wherein an upper clad layer is etched to mesa stripes to confine light in the horizontal direction, and a high-mesa-type waveguide wherein not only an upper clad layer but also a core layer and a lower clad layer are etched to mesa stripes.
Since the ridge-type waveguide has a structure not etching the core layer, the defect level of the side of the semiconductor optical element can be reduced. On the other hand, the high-mesa-type waveguide has features of larger light confinement in the horizontal direction and the reduction of electrical capacitance (for example, refer to Japanese Patent Application Laid-Open No. 2000-228558).
Since the characteristics of the ridge-type waveguide differ from the characteristics of the high-mesa-type waveguide, these waveguides can be combined in a semiconductor optical integrated element to improve overall characteristics. For example, in a semiconductor laser formed by integrating electric field absorbing optical modulators, the semiconductor laser section is formed of a ridge-type waveguide structure, and the optical modulator section is formed of a high-mesa-type waveguide structure. Since the defect level can be reduced by such a structure, laser characteristics can be improved in the laser section, the electric capacitance can be reduced in the optical modulator section, and high-speed modulation becomes possible.
In the above-described semiconductor optical integrated element, the optical mode distribution of a ridge-type waveguide differs from that of a high-mesa-type waveguide. Therefore, light is reflected at the boundary portion between the waveguides, and the characteristics of the semiconductor optical integrated element is deteriorated. For example, a semiconductor laser formed by integrating electric field absorbing optical modulators had a problem wherein the single-wavelength properties of the laser was deteriorated by the reflection of light at the boundary between the laser section and the optical modulator section.